Engine exhaust gas temperature control system

ABSTRACT

An engine exhaust gas temperature control system comprises a combustion chamber, a fuel supplying device, a secondary air supplying device and a control unit. The fuel supplying device is configured and arranged to supply fuel into an intake passage. The secondary air supplying device is configured and arranged to selectively supply secondary air to an exhaust passage. The control unit is configured to execute a secondary air combustion operation in which a valve overlapping period during which an intake valve and an exhaust valve are both open is set and the fuel supplying device is controlled to inject the fuel into the intake passage so that a portion of the fuel that blows by a combustion chamber and into the exhaust passage during the valve overlapping period is combusted by the secondary air supplied by the secondary air supplying device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2004-368905. The entire disclosure of Japanese Patent Application No. 2004-368905 is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an engine exhaust gas temperature control system configured and arranged to achieve early activation of an exhaust gas cleaning catalytic converter immediately after an engine is started.

2. Background Information

Japanese Laid-Open Patent Publication No. 2002-327619 discloses a conventional engine exhaust gas temperature control system comprising a fuel injection valve configured and arranged to inject fuel directly into a combustion chamber of an engine and a secondary air supplying component (air pump) configured and arranged to supply secondary air to an exhaust passage of the engine. In such conventional engine exhaust gas temperature control system, in addition to a main fuel injection that is combusted in a main combustion in the engine, the fuel injection valve also executes a supplementary fuel injection at a timing occurring generally between the power stroke and the exhaust stroke of the engine. The fuel injected with such supplementary injection is combusted in the exhaust passage of the engine with the secondary air supplied from the secondary air supplying component, thereby causing a temperature of the exhaust gas to rise. The conventional engine exhaust gas temperature control system is configured to raise the exhaust gas temperature in order to achieve earlier activation of an exhaust gas cleaning catalytic converter provided in the exhaust passage immediately after the engine is started.

In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved engine exhaust gas temperature control system. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.

SUMMARY OF THE INVENTION

The conventional engine exhaust gas temperature control system disclosed in the above mentioned reference is limited to be applied to a direct fuel injection engine configured to execute a supplementary fuel injection directly into a combustion chamber at a timing occurring generally between the power stroke and the exhaust stroke. Thus, the conventional engine exhaust gas temperature control system cannot be applied to an engine configured and arranged to supply fuel into an intake passage instead of directly into a combustion chamber. Moreover, in the conventional engine exhaust gas temperature control system, the control that is required to execute the two fuel injections is complex.

In an engine configured and arranged to supply the fuel into the intake passage, the temperature of the exhaust gas can be raised by increasing the amount of fuel supplied such that the air-fuel ratio of the air-fuel mixture inside the cylinder becomes rich. The rich air-fuel ratio results in a large amount of unburned fuel being discharged to the exhaust passage. By combusting this unburned fuel with secondary air supplied from a secondary air supplying component, the temperature of the exhaust gas can be raised. When this method is used to raise the exhaust gas temperature, the exhaust temperature raising effect becomes stronger the more the air-fuel ratio is richened.

However, when the air-fuel ratio becomes excessively rich, this method is inhibited by such problems as rich misfiring and poor combustion stability. Also, since there is a limit to how rich the air-fuel ratio can be to achieve acceptable exhaust emissions, this method does not allow the use of an air-fuel ratio that is optimum from the standpoint of exhaust emissions.

The present invention was conceived in view of these shortcomings of the conventional engine exhaust gas temperature control system. One object of the present invention is to provide an engine exhaust gas temperature control system that can be applied to an internal combustion engine configured to supply fuel to an intake passage or an intake port and can sufficiently raise the exhaust gas temperature while preventing rich misfiring by utilizing the secondary air in the exhaust passage in a highly effective manner.

In order to achieve the above mentioned object and other objects of the present invention, an engine exhaust gas temperature control system is provided that comprises a combustion chamber, a fuel supplying device, a secondary air supplying device and a control unit. The combustion chamber is fluidly coupled to an intake passage and an exhaust passage with an intake valve being disposed between the intake passage and the combustion chamber and an exhaust valve being disposed between the exhaust passage and the combustion chamber. The fuel supplying device is configured and arranged to supply fuel into the intake passage. The secondary air supplying device is configured and arranged to selectively supply secondary air to the exhaust passage. The control unit is configured to execute a secondary air combustion operation in which a valve overlapping period during which the intake valve and the exhaust valve are both open is set and the fuel supplying device is controlled to inject the fuel into the intake passage so that a portion of the fuel that blows by the combustion chamber and into the exhaust passage during the valve overlapping period is combusted by the secondary air supplied by the secondary air supplying device.

These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the attached drawings which form a part of this original disclosure:

FIG. 1 is an overall schematic system diagram of an engine exhaust gas temperature control system in accordance with a preferred embodiment of the present invention;

FIG. 2 is a flowchart executed in a control unit of the engine exhaust gas temperature control system for achieving early activation of a catalytic converter immediately after an engine is started in accordance with the preferred embodiment of the present invention;

FIG. 3 is a diagram showing valve lift characteristics of an intake valve and an exhaust valve in the engine exhaust gas temperature control system in accordance with the preferred embodiment of the present invention; and

FIG. 4 is a simplified diagrammatic cross sectional view of a cylinder head of an engine with the intake valve and the exhaust valve during an overlapping period illustrating a blow-by effect that occurs when a lift amount of the intake valve is small in the engine exhaust gas temperature control system in accordance with the preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Selected embodiments of the present invention will now be explained with reference to the drawings. It will be apparent to those skilled in the art from this disclosure that the following description of the embodiment of the present invention is provided for illustration only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.

FIG. 1 is an overall schematic system diagram of an engine exhaust gas temperature control system in accordance with a preferred embodiment of the present invention. As seen in FIG. 1, an engine 1 has an electronically controlled throttle valve 4 arranged in an intake passage 2 at a position near an inlet portion of an intake manifold 3. The throttle valve 4 is configured and arranged to control an intake air quantity of the engine 1. More specifically, the opening degree of the throttle valve 4 is controlled with a step motor (not shown) or the like based on a signal from an engine control unit (hereinafter called “ECU”) 20. Of course, it will be apparent to those skilled in the art from this disclosure that it is also acceptable to use a mechanical throttle valve that is mechanically operated with a wire or the like connected to an accelerator pedal as the throttle valve 4. The intake passage 2 preferably includes the intake manifold 3 and a plurality of intake ports 5 (only one intake port 5 is shown in FIG. 1) that are disposed downstream portions of the intake passage 2.

The engine 1 preferably includes a plurality of fuel injection valves 6 (only one fuel injection valve 6 is shown) so that each runner on an outlet side of the intake manifold 3 is provided with the fuel injection valve 6. Each of the fuel injection valves 6 is configured and arranged to inject fuel into the intake port 5 of the corresponding cylinder so that fuel stream is directed toward a head of an intake valve 7 of the cylinder coupled to a downstream portion of the intake port 5. Operations (opening and closing) of the fuel injection valves 6 are controlled by a solenoid that is electrically energized by an injection pulse signal issued from the ECU 20. The injection pulse signal is synchronized with an engine rotation, and a pulse width thereof determines a length of time that the fuel injection valves 6 remain open. The fuel injected from the fuel injection valves 6 is pressurized to a prescribed pressure.

When the intake valve 7 is opened, intake air that has been controlled by the throttle valve 4 and fuel that has been injected by the fuel injection valve 6 are drawn into a combustion chamber 10 of the engine 1.

As seen in FIG. 1, the engine 1 is provided with a variable valve operating angle and lift amount control device (i.e., a continuously variable valve event and lift control device (VEL)) 8 and a variable valve timing control device (i.e., a variable timing control device (VTC)) 9. The VEL 8 and the VTC 9 are provided as a variable valve operating device configured and arranged to change the lift characteristic of the intake valve 7. More specifically, the VEL 8 is configured and arranged to change an operating angle and a lift amount of the intake valve 7 in a continuously variable manner while keeping a lift center angle of the intake valve 7 (i.e., the center angle phase of the working angle of the intake valve 7 (open duration)) substantially constant by varying a posture of a link that mechanically connects a camshaft (not shown) and a cam (not shown) of the engine 1 together. The VTC 9 is configured and arranged to change the lift center angle of the intake valve 7 in a continuously variable manner by changing a rotational phase between a crankshaft (not shown) and the camshaft of the engine 1. The VEL and the VTC are conventional components that are well known in the art. Since the VEL and the VTC are well known in the art, these structures will not be discussed or illustrated in detail herein. Moreover, it will be apparent to those skilled in the art from this disclosure that any type of variable valve operating device can be used to carry out the present invention.

The intake air and the fuel that are drawn into the combustion chamber 10 form an air-fuel mixture that is ignited and burned by a spark from a spark plug 11. The spark plug 11 is configured to produce a spark at an ignition timing controlled by the ECU 20. The exhaust gas that remains after combustion is discharged through an exhaust valve 12 to an exhaust passage 13. An exhaust gas cleaning catalytic converter 14 is provided in the exhaust gas passage 13 as seen in FIG. 1.

The engine 1 is further provided with a secondary air supply passage 16 inside a cylinder head at a position along the exhaust passage 13 that is upstream of the exhaust gas cleaning catalytic converter 14. More specifically, the secondary air supply passage 16 is positioned so that secondary air is supplied to an exhaust port 15 of the corresponding cylinder as the secondary air being directed toward a head of the exhaust valve 12 coupled to an upstream portion of the exhaust port 15. An electric powered air pump 17 is provided for supplying the secondary air. The discharge side of the air pump 17 is connected to the secondary air supply passages 16 through a pipe 19 in which a shut-off valve 18 is installed. As shown in FIG. 1, a battery B serves as a power supply for the air pump 17. The air pump 17 preferably constitutes at least part of a secondary air supplying device of the present invention.

The ECU 20 is configured to receive input signals from various sensors including a signal indicating an accelerator position APO detected by an accelerator pedal sensor 21, a signal indicating an engine rotational speed Ne detected by a crank angle sensor 22, a signal indicating an intake air quantity Qa detected by an air flow meter 23, a signal indicating an engine coolant temperature Tw detected by a coolant temperature sensor 24, a signal indicating an exhaust gas air-fuel ratio detected by an air-fuel ratio sensor 25, and a signal indicating a catalytic converter temperature Tc detected by a catalytic converter temperature sensor 26. The ECU 20 is also configured to receive signals from an engine key switch 27 having an ignition switch 27 a and a start switch 27 b.

Based on the engine operating conditions indicated by these input signals, the ECU 20 is configured to control the opening degree of the throttle valve 4, the fuel injection timing and the fuel injection quantity of the fuel injection valves 6, and the ignition timing of the spark plugs 11. The ECU 20 is also configured to control operations of the VEL 8, the VTC 9, the air pump 17, and the shut-off valve 18.

The ECU 20 preferably includes a microcomputer with an engine exhaust gas temperature control program that controls the exhaust gas temperature immediately after the engine 1 is started as discussed below. The ECU 20 can also include other conventional components such as an input interface circuit, an output interface circuit, and storage devices such as a ROM (Read Only Memory) device and a RAM (Random Access Memory) device. The microcomputer of the ECU 20 is programmed to control the fuel injection valves 6, the VEL 8, the VTC 9, the air pump 17, the shut-off valve 18, and other components of the engine 1 operatively coupled to the ECU 20. The memory circuit stores processing results and control programs such as ones for the engine exhaust gas temperature control operations that are run by the processor circuit. The ECU 20 is operatively coupled to the various sensors and components the engine 1 in a conventional manner. The internal RAM of the ECU 20 stores statuses of operational flags and various control data. The internal ROM of the ECU 20 stores the maps and data for various operations. The ECU 20 is capable of selectively controlling any of the components of the control system in accordance with the control program. It will be apparent to those skilled in the art from this disclosure that the precise structure and algorithms for the ECU 20 can be any combination of hardware and software that will carry out the functions of the present invention. In other words, “means plus function” clauses as utilized in the specification and claims should include any structure or hardware and/or algorithm or software that can be utilized to carry out the function of the “means plus function” clause.

Referring now to a flowchart of FIG. 2, an engine exhaust gas temperature control executed in the ECU 20 for achieving early activation of the exhaust gas cleaning catalytic converter 14 immediately after the engine 1 is started will now be described. This control routine shown in FIG. 2 is preferably executed when an engine cranking starts and is synchronized based on either time or rotation of the engine 1. In other words, the cycle time of the control routine shown in FIG. 2 is controlled based on either a clock or the rotation of the engine 1 as indicated based on the crank angle sensor 22.

In step S1, the ECU 20 is configured to determine if the engine 1 is being cranked, i.e., if the start switch 27 b of the engine key switch 27 is ON. If the engine 1 is being cranked in step S1, the ECU 20 is configured to execute the processing in steps S2 to S4.

In step S2, the ECU 20 is configured to turn the secondary air supply OFF. More specifically, the ECU 20 is configured to control operations of the air pump 17 and the shut-off valve 18 so that the air pump 17 for supplying the secondary air is held in a stopped condition and the shut-off valve 18 is held closed in step S2. The secondary air supply is turned OFF in step S2 to curb consumption of electric power from the battery B because the engine 1 is being cranked with the starter motor that is driven by the electric power from the battery B.

In step S3, the ECU 20 is configured to set a lift characteristic of the intake valve 7 to a normal characteristic. The normal characteristic corresponds to the characteristic indicated in FIG. 3 as “secondary air supply OFF characteristic”. As seen in FIG. 3, the normal characteristic (the secondary air supply OFF characteristic) has a larger valve working angle, a larger lift amount, and a later lift center angle than a characteristic indicated as “secondary air supply ON characteristic”. Moreover, when the intake valve 7 is operated with the normal characteristic, in the vicinity of TDC, there is a slight period of overlap (i.e., a valve overlapping period) during which both the intake valve 7 and the exhaust valve 12 are open.

In step S4, the ECU 20 is configured to set a fuel injection timing IT to an engine starting timing and to set a fuel injection quantity Ti to an engine starting value, respectively. The ECU 20 then returns to the beginning of the control routine. The engine starting timing of the fuel injection timing IT and the engine starting value of the fuel injection quantity Ti are preferably set to values that are appropriate for a condition in which the engine 1 is starting.

If the ECU 20 determines that the engine 1 is not being cranked in step S1, the ECU 20 proceeds to step S5.

In step S5, the ECU 20 is configured to determine if a temperature increase condition is satisfied which indicates it is necessary to raise the temperature of the catalytic converter 14. More specifically, the ECU 20 is configured to read the catalytic converter temperature Tc detected by the catalytic converter temperature sensor 26 and to determine if the temperature Tc is lower than a prescribed activation temperature. It is also acceptable to configure the ECU 20 to estimate the catalytic converter temperature based on the operating conditions of the engine 1 and to determine if a catalyst in the catalytic converter 14 is active or not active. If the ECU 20 determines the temperature increase condition is satisfied, i.e., if the catalytic converter is not active, in step S5, the ECU 20 proceeds to step S6.

In step S6, the ECU 20 is configured to turn the secondary air supply ON. More specifically, the ECU 20 is configured to control operations of the air pump 17 and the shut-off valve 18 so that the air pump 17 for supplying the secondary air is operated and the shut-off valve 18 is opened. When the air pump 17 is started, a prescribed delay period (delay time) elapses before a discharge flow rate of the air pump 17 (flow rate of secondary air from the secondary air supply passage 16 to the exhaust port 15) reaches a steady-state flow rate.

Thus, in step S7, the ECU 20 is configured to compare a value of a timer T for measuring an amount of time elapsed since the air pump 17 was started to a prescribed amount of time Tht corresponding to an amount of time required for the discharge flow rate of the air pump 17 to reach the steady-state flow rate. In other words, in step S7, the ECU 20 is configured to determine if the value of the timer T is less than the prescribed amount of time Tht. If the value of the timer T is less than the prescribed amount of time Tht in step S7, the ECU 20 is configured to execute the processing in steps S8 and S9. The ECU 20 is configured to execute steps S8 and S9 until the secondary air is supplied to the exhaust port 15 in a stable manner (i.e., until the discharge flow rate of the air pump 17 reaches the steady-state flow rate).

In step S8, the ECU 20 is configured to set the lift characteristic of the intake valve 7 to the normal characteristic.

In step S9, the ECU 20 is configured to set the fuel injection timing IT to a normal timing and the fuel injection quantity Ti to a slightly rich value. Then, the ECU 20 is configured to return to the beginning of the control routine.

The normal timing for the fuel injection timing IT is indicated as “secondary air supply OFF timing” in FIG. 3 and is set such that the fuel is injected only during an exhaust stroke and not during the valve overlapping period (i.e., the fuel injection ends before the intake valve 7 opens). In other words, with the normal timing for the fuel injection timing IT (the secondary air supply OFF timing in FIG. 3), the fuel is injected such that there is no blow-by of fuel to the exhaust port 15 through the combustion chamber 10 because, although the air pump 17 is operating, there is still little supply of secondary air to the exhaust port 15. In other words, since the air pump 17 still has not reached full speed (full output), and thus, the supply of the secondary air to the exhaust port 15 is still small, the fuel injection timing is set to the normal fuel injection timing IT to prevent the blow-by of fuel into the exhaust port 15.

The slightly rich fuel quantity Ti set to such a value as to keep the air-fuel ratio inside the cylinder down to such a level that rich misfiring does not occur since the injected fuel is not being blown by to the exhaust port 15.

If the value of the timer T is equal to or larger than the value of the prescribed amount of time Tht in step S7, i.e., when the supply of secondary air to the exhaust port 15 has stabilized, the ECU 20 proceeds to the processing in steps S10 and S11.

In step S10, the ECU 20 is configured to set the lift characteristic of the intake valve 7 such that the valve overlapping period is extended. In other words, the valve characteristic of the intake valve 7 is changed from the characteristic indicated as “secondary air supply OFF characteristic” in FIG. 3 to the characteristic indicated as “secondary air supply ON characteristic” (indicated with a chain line). More specifically, the ECU 20 is configured to control the VTC 9 to advance the lift center angle of the intake valve 7 and thereby increasing the length of the valve overlapping period during which both the intake valve 7 and the exhaust valve 12 are open. Simultaneously, the ECU 20 is configured to control the VEL 8 to reduce the valve lift amount of the intake valve 7. However, since reducing the lift amount of the intake valve 7 also reduces the working angle of the intake valve 7, the valve lift amount of the intake valve 7 is controlled such that the reduction in the working angle is smaller than the reduction in the valve lift amount.

In step S11, the ECU 20 is configured to set the fuel injection timing IT to a blow-by timing and the fuel injection quantity Ti to a strongly rich value. The ECU 20 is then configured to return to the beginning of the control routine.

In other words, in step S11, the fuel injection timing IT is changed from the normal timing indicated as “secondary air supply OFF timing” to the blow-by timing indicated as “secondary air supply ON timing” in FIG. 3. More specifically, the fuel injection timing IT is retarded with respect to the normal timing (which ends before the valve overlapping period starts such that the entire fuel injection occurs during the exhaust stroke). Thus, when the fuel injection timing IT is set to the blow-by timing (the secondary air supply ON in FIG. 3), at least a portion of the fuel injection takes place during the valve overlapping period (i.e., a portion of the fuel injection period occurs simultaneously with the valve overlapping period) and a portion of the injected fuel blows by the combustion chamber 10 and into the exhaust port 15. In this preferred embodiment of the present invention, the fuel injection is executed to extend from a point before the valve overlapping period starts to a point after the valve overlapping period ends as shown in FIG. 3.

Also in step S11, the fuel injection quantity Ti is set to such a strongly rich value that rich misfiring would occur if a portion of the fuel did not blow-by to the exhaust port 15.

Thus, in steps S10 and S11, the ECU 20 is configured to set the intake valve lift characteristic such that the valve overlapping period during which the intake valve 7 and the exhaust valve 12 are both open is extended (enlarged) and to set the fuel injection characteristic such that fuel is injected during the valve overlapping period, thereby causing a portion of the fuel injected into the intake port 5 to blow-by the combustion chamber 10 and into the exhaust port 15. With this arrangement, the blow-by of fuel into the exhaust port 15 is induced aggressively and the blown-by fuel is burned with the secondary air. Thus, the air fuel ratio inside the cylinder is held to a level where rich misfiring does not occur even if the air-fuel ratio is set to such a strongly rich ratio that the engine 1 would misfire under conditions where fuel was not blowing by to the exhaust port 15. Consequently, the stability of the combustion can be ensured while achieving a sufficiently rich air-fuel ratio inside the exhaust port 15. As a result, by utilizing the secondary air in the exhaust port 15 in a highly effective manner to burn the fuel blown into the exhaust port 15, the exhaust gas temperature can be raised enough to achieve early activation of the catalytic converter 14 immediately after the engine 1 is started. The processing in steps S10 and S11 preferably constitute a secondary air combustion operation of the present invention.

Since the lift amount of the intake valve 7 during the blow-by setting (i.e., the secondary air supply ON characteristic) is controlled to be smaller than the normal lift characteristic, the positioning of the intake valve 7 and the exhaust valve 12 is, for example, as shown in FIG. 4 when the fuel is blowing by. As seen in FIG. 4, the head and a seat surface of the intake valve 7 forms a flow of air (indicated as F in FIG. 4) that flows across the combustion chamber 10 toward the exhaust valve 12 and the exhaust port 15. By placing the fuel into this flow of air, the fuel can be made to blow-by the combustion chamber 10 and into the exhaust port 15 in an effective manner.

On the other hand, if the ECU 20 determines in step S5 that the temperature increase condition is not satisfied, i.e., if the catalytic converter 14 is active, the ECU 20 is configured to execute the processing in steps S12 to S14.

In step S12, the ECU 20 is configured to turn the secondary air supply OFF. More specifically, the ECU 20 is configured to stop the secondary air pump 17 and to close the shut-off valve 18.

In step S13, the ECU 20 is configured to set the lift characteristic of the intake valve 7 to the normal characteristic.

In step S14, the ECU 20 is configured to set the fuel injection timing IT to the normal timing and the fuel injection quantity Ti to the normal quantity. The ECU 20 is then configured to return to the beginning of the control routine.

Although in the embodiment described heretofore the fuel injection is executed during the valve overlapping period while the air pump 17 is operated, it is not mandatory to inject fuel during the valve overlapping period to carry out the present invention. The engine exhaust gas temperature control system of the present invention can be configured and arranged to control the valve lift characteristic of the intake valve 7 and the fuel injection characteristic of the fuel injection valve 6 so that even if the fuel injection is ended before the valve overlapping period is reached, some of the fuel injected into the intake port 5 blows by to the exhaust port 15 when the valve overlapping period occurs.

Also, although the preferred embodiment explained above presents a case in which the temperature of the exhaust gas needs to be raised in order to achieve early activation of the catalytic converter immediately after the engine 1 is started, substantially the same control can also be used to raise the temperature of the exhaust gas for other purposes, such as raising the temperature of the exhaust gas in order to remove sulfur contamination (“poisoning”) from the catalytic converter 14.

Moreover, in the preferred embodiment explained above, the variable valve operating device (the VEL 8 and the VTC 9) is configured and arranged to control the valve characteristic of the intake valve 7 to change the valve overlapping period. Of course, it will be apparent to those skilled in the art from this disclosure that the engine exhaust gas temperature control system of the present invention can use a variable valve operating device that is configured and arranged to control the valve characteristic of the exhaust valve 12 and/or the intake valve 7 to vary the valve overlapping period to carry out the present invention.

Accordingly, the engine exhaust gas control system in accordance with the present invention is configured and arranged such that a portion of the fuel supplied to the intake port 5 blows by the combustion chamber 10 and into the exhaust port 15 during the valve overlapping period or such that fuel is injected into the intake port 5 during the valve overlapping period. Therefore, the blow-by of the fuel into the exhaust passage 13 is more aggressively promoted, and the blown by fuel is combusted with the secondary air supplied by the air pump 17. Consequently, even if the air-fuel ratio is set to such a strongly rich ratio that the engine 1 would misfire if fuel blow-by was not occurring, the air-fuel ratio inside the cylinder can be held to a level where misfiring does not occur because not all of the fuel supplied remains inside the cylinder. As a result, stable combustion can be ensured while the secondary air can be utilized in a highly effective manner in the exhaust passage 13 to raise the exhaust gas temperature sufficiently to achieve earlier activation of the catalytic converter 14 immediately after the engine 1 is started. Thus, the engine exhaust gas temperature control system of the present invention is configured to enable the exhaust temperature to be raised enough to achieve early activation of the catalytic converter 14 immediately after the engine 1 is started while preventing the occurrence of rich misfiring inside the combustion chamber 10.

The term “detect” as used herein to describe an operation or function carried out by a component, a section, a device or the like includes a component, a section, a device or the like that does not require physical detection, but rather includes determining, measuring, modeling, predicting or computing or the like to carry out the operation or function. The term “configured” as used herein to describe a component, section or part of a device includes hardware and/or software that is constructed and/or programmed to carry out the desired function. Moreover, terms that are expressed as “means-plus function” in the claims should include any structure that can be utilized to carry out the function of that part of the present invention. The terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. For example, these terms can be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the word it modifies.

While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. Furthermore, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. Thus, the scope of the invention is not limited to the disclosed embodiments. 

1. An engine exhaust gas temperature control system comprising: a combustion chamber fluidly coupled to an intake passage and an exhaust passage with an intake valve being disposed between the intake passage and the combustion chamber and an exhaust valve being disposed between the exhaust passage and the combustion chamber; a fuel supplying device configured and arranged to supply fuel into the intake passage; a secondary air supplying device configured and arranged to selectively supply secondary air to the exhaust passage; and a control unit configured to execute a secondary air combustion operation in which a valve overlapping period during which the intake valve and the exhaust valve are both open is set and the fuel supplying device is controlled to inject the fuel into the intake passage so that a portion of the fuel that blows by the combustion chamber and into the exhaust passage during the valve overlapping period is combusted by the secondary air supplied by the secondary air supplying device.
 2. The engine exhaust gas temperature control system recited in claim 1, further comprising a variable valve operating device operatively coupled to the control unit, and configured and arranged to change the valve overlapping period by changing a lift characteristic of at least one of the intake valve and the exhaust valve, the control unit being further configured to control the variable valve operating device so that the valve overlapping period during operation of the secondary air supplying device is longer than the valve overlapping period when the secondary air supplying device is not operating.
 3. The engine exhaust gas temperature control system recited in claim 2, wherein the control unit is configured to control the variable valve operating device so that a lift amount of the intake valve during operation of the secondary air supplying device is smaller than the lift amount of the intake valve when the secondary air supplying device is not operating.
 4. The engine exhaust gas temperature control system recited in claim 1, further comprising an exhaust gas cleaning catalytic converter provided in the exhaust passage, the control unit being further configured to operate the secondary air supplying device and to execute the secondary air combustion operation upon a temperature increase condition is satisfied.
 5. The engine exhaust gas temperature control system recited in claim 1, wherein the fuel supplying device includes a fuel injection valve that is configured and arranged to inject fuel into an intake port of the intake passage, the control unit is further configured to control the fuel injection valve to inject the fuel into the intake port during at least part of the valve overlapping period.
 6. The engine exhaust gas temperature control system recited in claim 5, wherein the control unit is further configured to control the fuel injection valve to inject the fuel into the intake port from a timing before the valve overlapping period starts to a timing after the valve overlapping period ends.
 7. The engine exhaust gas temperature control system recited in claim 5, wherein the control unit is further configured to prohibit the fuel injection valve from injecting fuel during the valve overlapping period until a prescribed amount of time elapses after an operation of the secondary air supplying device is started.
 8. The engine exhaust gas temperature control system recited in claim 5, wherein the control unit is further configured to prohibit the secondary air combustion operation until a prescribed amount of time elapses after an operation of the secondary air supplying device is started.
 9. The engine exhaust gas temperature control system recited in claim 5, further comprising a variable valve operating device operatively coupled to the control unit, and configured and arranged to change the valve overlapping period by changing a lift characteristic of at least one of the intake valve and the exhaust valve, the control unit being further configured to control the variable valve operating device so that the valve overlapping period during operation of the secondary air supplying device is longer than the valve overlapping period when the secondary air supplying device is not operating.
 10. The engine exhaust gas temperature control system recited in claim 9, wherein the control unit is configured to control the variable valve operating device so that a lift amount of the intake valve during operation of the secondary air supplying device is smaller than the lift amount of the intake valve when the secondary air supplying device is not operating.
 11. The engine exhaust gas temperature control system recited in claim 5, further comprising an exhaust gas cleaning catalytic converter provided in the exhaust passage, the control unit being further configured to operate the secondary air supplying device and to execute the secondary air combustion operation upon a temperature increase condition is satisfied.
 12. The engine exhaust gas temperature control system recited in claim 11, wherein the control unit is further configured to prohibit the secondary air combustion operation until a prescribed amount of time elapses after an operation of the secondary air supplying device is started.
 13. A method of controlling an engine exhaust gas temperature comprising: providing a combustion chamber fluidly coupled to an intake passage and an exhaust passage with an intake valve being disposed between the intake passage and the combustion chamber and an exhaust valve being disposed between the exhaust passage and the combustion chamber; supplying secondary air to the exhaust passage; setting an valve overlapping period during which the intake valve and the exhaust valve are both open; and supplying fuel into the intake passage so that a portion of the fuel that blows by the combustion chamber and into the exhaust passage during the valve overlapping period is combusted by the secondary air in the exhaust passage.
 14. The method as recited in claim 13, wherein the setting of the valve overlapping period is performed by controlling a lift characteristic of at least one of the intake valve and the exhaust valve.
 15. The method as recited in claim 13, wherein the supplying of the fuel into the intake passage is performed by supplying the fuel into an intake port of the intake passage during the valve overlapping period.
 16. An engine exhaust gas temperature control system comprising: means for providing a combustion chamber fluidly coupled to an intake passage and an exhaust passage with an intake valve being disposed between the intake passage and the combustion chamber and an exhaust valve being disposed between the exhaust passage and the combustion chamber; secondary air supplying means for supplying secondary air to the exhaust passage; valve overlapping period setting means for setting an valve overlapping period during which the intake valve and the exhaust valve are both open; and fuel supplying means for supplying fuel into the intake passage so that a portion of the fuel that blows by the combustion chamber and into the exhaust passage during the valve overlapping period is combusted by the secondary air in the exhaust passage.
 17. The engine exhaust gas temperature control system recited in claim 16, wherein the valve overlapping period setting means further includes a function for controlling a lift characteristic of at least one of the intake valve and the exhaust valve for setting the valve overlapping period.
 18. The engine exhaust gas temperature control system recited in claim 16, wherein the fuel supplying means further includes a function for supplying the fuel into an intake port of the intake passage during the valve overlapping period. 